DcValueVoltageInitializer fails in presence of resistive only branches

src/main/java/com/powsybl/openloadflow/OpenLoadFlowProvider.java

@@ -22,7 +22,10 @@
 import com.powsybl.math.matrix.MatrixFactory;
 import com.powsybl.math.matrix.SparseMatrixFactory;
 import com.powsybl.openloadflow.ac.nr.NewtonRaphsonStatus;
-import com.powsybl.openloadflow.ac.outerloop.*;
+import com.powsybl.openloadflow.ac.outerloop.AcLoadFlowParameters;
+import com.powsybl.openloadflow.ac.outerloop.AcLoadFlowResult;
+import com.powsybl.openloadflow.ac.outerloop.AcloadFlowEngine;
+import com.powsybl.openloadflow.ac.outerloop.OuterLoop;
 import com.powsybl.openloadflow.dc.DcLoadFlowEngine;
 import com.powsybl.openloadflow.dc.DcLoadFlowResult;
 import com.powsybl.openloadflow.graph.EvenShiloachGraphDecrementalConnectivityFactory;
@@ -38,7 +41,6 @@
 import com.powsybl.tools.PowsyblCoreVersion;
 import org.jgrapht.Graph;
 import org.jgrapht.alg.interfaces.SpanningTreeAlgorithm;
-import org.jgrapht.alg.spanning.KruskalMinimumSpanningTree;
 import org.slf4j.Logger;
 import org.slf4j.LoggerFactory;
 
@@ -127,7 +129,7 @@ private LoadFlowResult runAc(Network network, LoadFlowParameters parameters, Rep
                 result.getNetwork().updateState(updateParameters);
 
                 // zero or low impedance branch flows computation
-                computeZeroImpedanceFlows(result.getNetwork());
+                computeZeroImpedanceFlows(result.getNetwork(), false);
             }
 
             LoadFlowResult.ComponentResult.Status status;
@@ -159,12 +161,10 @@ private LoadFlowResult runAc(Network network, LoadFlowParameters parameters, Rep
         return new LoadFlowResultImpl(ok, Collections.emptyMap(), null, componentResults);
     }
 
-    private void computeZeroImpedanceFlows(LfNetwork network) {
-        Graph<LfBus, LfBranch> zeroImpedanceSubGraph = network.createZeroImpedanceSubGraph();
-        if (!zeroImpedanceSubGraph.vertexSet().isEmpty()) {
-            SpanningTreeAlgorithm.SpanningTree<LfBranch> spanningTree = new KruskalMinimumSpanningTree<>(zeroImpedanceSubGraph).getSpanningTree();
-            new ZeroImpedanceFlows(zeroImpedanceSubGraph, spanningTree).compute();
-        }
+    private void computeZeroImpedanceFlows(LfNetwork network, boolean dc) {
+        Graph<LfBus, LfBranch> zeroImpedanceSubGraph = network.getZeroImpedanceSubGraph(dc);
+        SpanningTreeAlgorithm.SpanningTree<LfBranch> spanningTree = network.getZeroImpedanceSpanningTree(dc);
+        new ZeroImpedanceFlows(zeroImpedanceSubGraph, spanningTree, dc).compute();
     }
 
     private LoadFlowResult runDc(Network network, LoadFlowParameters parameters, Reporter reporter) {
@@ -197,7 +197,7 @@ private LoadFlowResult.ComponentResult processResult(Network network, DcLoadFlow
             result.getNetwork().updateState(updateParameters);
 
             // zero or low impedance branch flows computation
-            computeZeroImpedanceFlows(result.getNetwork());
+            computeZeroImpedanceFlows(result.getNetwork(), true);
         }
 
         return new LoadFlowResultImpl.ComponentResultImpl(

src/main/java/com/powsybl/openloadflow/ac/equations/AcEquationSystemCreator.java

@@ -249,8 +249,8 @@ private List<EquationTerm<AcVariableType, AcEquationType>> createReactiveTerms(L
         List<EquationTerm<AcVariableType, AcEquationType>> terms = new ArrayList<>();
         for (LfBranch branch : controllerBus.getBranches()) {
             EquationTerm<AcVariableType, AcEquationType> q;
-            if (branch.isZeroImpedance()) {
-                if (!branch.isSpanningTreeEdge()) {
+            if (branch.isZeroImpedance(false)) {
+                if (!branch.isSpanningTreeEdge(false)) {
                     continue;
                 }
                 if (branch.getBus1() == controllerBus) {
@@ -739,8 +739,8 @@ private static void createHvdcAcEmulationEquations(LfHvdc hvdc, EquationSystem<A
     private void createBranchEquations(LfBranch branch,
                                        EquationSystem<AcVariableType, AcEquationType> equationSystem) {
         // create zero and non zero impedance branch equations
-        if (branch.isZeroImpedance()) {
-            if (branch.isSpanningTreeEdge()) {
+        if (branch.isZeroImpedance(false)) {
+            if (branch.isSpanningTreeEdge(false)) {
                 createNonImpedantBranch(branch, branch.getBus1(), branch.getBus2(), equationSystem);
             }
         } else {

src/main/java/com/powsybl/openloadflow/ac/equations/AcEquationSystemUpdater.java

@@ -16,7 +16,7 @@
 public class AcEquationSystemUpdater extends AbstractEquationSystemUpdater<AcVariableType, AcEquationType> {
 
     public AcEquationSystemUpdater(EquationSystem<AcVariableType, AcEquationType> equationSystem) {
-        super(equationSystem);
+        super(equationSystem, false);
     }
 
     @Override

src/main/java/com/powsybl/openloadflow/dc/equations/DcEquationSystemCreator.java

@@ -115,8 +115,8 @@ private void createBranches(EquationSystem<DcVariableType, DcEquationType> equat
         for (LfBranch branch : network.getBranches()) {
             LfBus bus1 = branch.getBus1();
             LfBus bus2 = branch.getBus2();
-            if (branch.isZeroImpedance()) {
-                if (branch.isSpanningTreeEdge()) {
+            if (branch.isZeroImpedance(true)) {
+                if (branch.isSpanningTreeEdge(true)) {
                     createNonImpedantBranch(equationSystem, branch, bus1, bus2);
                 }
             } else {

src/main/java/com/powsybl/openloadflow/dc/equations/DcEquationSystemUpdater.java

@@ -16,7 +16,7 @@
 public class DcEquationSystemUpdater extends AbstractEquationSystemUpdater<DcVariableType, DcEquationType> {
 
     public DcEquationSystemUpdater(EquationSystem<DcVariableType, DcEquationType> equationSystem) {
-        super(equationSystem);
+        super(equationSystem, true);
     }
 
     @Override

src/main/java/com/powsybl/openloadflow/lf/AbstractEquationSystemUpdater.java

@@ -22,8 +22,11 @@ public abstract class AbstractEquationSystemUpdater<V extends Enum<V> & Quantity
 
     protected final EquationSystem<V, E> equationSystem;
 
-    protected AbstractEquationSystemUpdater(EquationSystem<V, E> equationSystem) {
+    protected final boolean dc;
+
+    protected AbstractEquationSystemUpdater(EquationSystem<V, E> equationSystem, boolean dc) {
         this.equationSystem = equationSystem;
+        this.dc = dc;
     }
 
     protected static void checkSlackBus(LfBus bus, boolean disabled) {
@@ -35,7 +38,7 @@ protected static void checkSlackBus(LfBus bus, boolean disabled) {
     protected abstract void updateNonImpedantBranchEquations(LfBranch branch, boolean enable);
 
     protected void updateElementEquations(LfElement element, boolean enable) {
-        if (element instanceof LfBranch && ((LfBranch) element).isZeroImpedance() && ((LfBranch) element).isSpanningTreeEdge()) {
+        if (element instanceof LfBranch && ((LfBranch) element).isZeroImpedance(dc) && ((LfBranch) element).isSpanningTreeEdge(dc)) {
             updateNonImpedantBranchEquations((LfBranch) element, enable);
         } else {
             // update all equations related to the element

src/main/java/com/powsybl/openloadflow/network/LfBranch.java

@@ -139,11 +139,11 @@ default List<LfLimit> getLimits2(LimitType type) {
 
     double computeApparentPower2();
 
-    boolean isZeroImpedance();
+    boolean isZeroImpedance(boolean dc);
 
-    void setSpanningTreeEdge(boolean spanningTreeEdge);
+    void setSpanningTreeEdge(boolean dc, boolean spanningTreeEdge);
 
-    boolean isSpanningTreeEdge();
+    boolean isSpanningTreeEdge(boolean dc);
 
     Evaluable getA1();
 
@@ -174,5 +174,5 @@ static double getDiscretePhaseControlTarget(LfBranch branch, DiscretePhaseContro
 
     boolean isConnectedAtBothSides();
 
-    void setMinZ(boolean dc, double lowImpedanceThreshold);
+    void setMinZ(double lowImpedanceThreshold);
 }

src/main/java/com/powsybl/openloadflow/network/LfNetwork.java

@@ -15,6 +15,8 @@
 import com.powsybl.openloadflow.util.PerUnit;
 import com.powsybl.openloadflow.util.Reports;
 import org.jgrapht.Graph;
+import org.jgrapht.alg.interfaces.SpanningTreeAlgorithm;
+import org.jgrapht.alg.spanning.KruskalMinimumSpanningTree;
 import org.jgrapht.graph.Pseudograph;
 import org.slf4j.Logger;
 import org.slf4j.LoggerFactory;
@@ -75,6 +77,14 @@ public class LfNetwork extends AbstractPropertyBag implements PropertyBag {
 
     private GraphConnectivity<LfBus, LfBranch> connectivity;
 
+    private Graph<LfBus, LfBranch> dcZeroImpedanceSubGraph;
+
+    private Graph<LfBus, LfBranch> acZeroImpedanceSubGraph;
+
+    private SpanningTreeAlgorithm.SpanningTree<LfBranch> dcZeroImpedanceSpanningTree;
+
+    private SpanningTreeAlgorithm.SpanningTree<LfBranch> acZeroImpedanceSpanningTree;
+
     private Reporter reporter;
 
     public LfNetwork(int numCC, int numSC, SlackBusSelector slackBusSelector,
@@ -120,13 +130,21 @@ public void updateSlack() {
         }
     }
 
+    private void invalidateZeroImpedanceSubGraphs() {
+        dcZeroImpedanceSubGraph = null;
+        acZeroImpedanceSubGraph = null;
+        dcZeroImpedanceSpanningTree = null;
+        acZeroImpedanceSpanningTree = null;
+    }
+
     public void addBranch(LfBranch branch) {
         Objects.requireNonNull(branch);
         branch.setNum(branches.size());
         branches.add(branch);
         branchesById.put(branch.getId(), branch);
         invalidateSlack();
         connectivity = null;
+        invalidateZeroImpedanceSubGraphs();
 
         // create bus -> branches link
         if (branch.getBus1() != null) {
@@ -447,10 +465,10 @@ public void reportBalance(Reporter reporter) {
             this, activeGeneration, activeLoad, reactiveGeneration, reactiveLoad);
     }
 
-    public void fix(boolean minImpedance, boolean dc, double lowImpedanceThreshold) {
+    public void fix(boolean minImpedance, double lowImpedanceThreshold) {
         if (minImpedance) {
             for (LfBranch branch : branches) {
-                branch.setMinZ(dc, lowImpedanceThreshold);
+                branch.setMinZ(lowImpedanceThreshold);
             }
         }
     }
@@ -496,7 +514,7 @@ public static <T> List<LfNetwork> load(T network, LfNetworkLoader<T> networkLoad
         List<LfNetwork> lfNetworks = networkLoader.load(network, parameters, reporter);
         for (LfNetwork lfNetwork : lfNetworks) {
             Reporter reporterNetwork = Reports.createPostLoadingProcessingReporter(lfNetwork.getReporter());
-            lfNetwork.fix(parameters.isMinImpedance(), parameters.isDc(), parameters.getLowImpedanceThreshold());
+            lfNetwork.fix(parameters.isMinImpedance(), parameters.getLowImpedanceThreshold());
             lfNetwork.validate(parameters.isDc(), reporterNetwork);
             if (lfNetwork.isValid()) {
                 lfNetwork.reportSize(reporterNetwork);
@@ -508,25 +526,60 @@ public static <T> List<LfNetwork> load(T network, LfNetworkLoader<T> networkLoad
         return lfNetworks;
     }
 
+    private static SpanningTreeAlgorithm.SpanningTree<LfBranch> createZeroImpedanceSpanningTree(Graph<LfBus, LfBranch> zeroImpedanceSubGraph, boolean dc) {
+        if (!zeroImpedanceSubGraph.vertexSet().isEmpty()) {
+            SpanningTreeAlgorithm.SpanningTree<LfBranch> spanningTree = new KruskalMinimumSpanningTree<>(zeroImpedanceSubGraph).getSpanningTree();
+            for (LfBranch branch : spanningTree.getEdges()) {
+                branch.setSpanningTreeEdge(dc, true);
+            }
+            return spanningTree;
+        }
+        return null;
+    }
+
+    public void updateZeroImpedanceCache(boolean dc) {
+        if (dc) {
+            if (dcZeroImpedanceSubGraph == null) {
+                dcZeroImpedanceSubGraph = createZeroImpedanceSubGraph(true);
+                dcZeroImpedanceSpanningTree = createZeroImpedanceSpanningTree(dcZeroImpedanceSubGraph, true);
+            }
+        } else {
+            if (acZeroImpedanceSubGraph == null) {
+                acZeroImpedanceSubGraph = createZeroImpedanceSubGraph(false);
+                acZeroImpedanceSpanningTree = createZeroImpedanceSpanningTree(acZeroImpedanceSubGraph, false);
+            }
+        }
+    }
+
+    public Graph<LfBus, LfBranch> getZeroImpedanceSubGraph(boolean dc) {
+        updateZeroImpedanceCache(dc);
+        return dc ? dcZeroImpedanceSubGraph : acZeroImpedanceSubGraph;
+    }
+
+    public SpanningTreeAlgorithm.SpanningTree<LfBranch> getZeroImpedanceSpanningTree(boolean dc) {
+        updateZeroImpedanceCache(dc);
+        return dc ? dcZeroImpedanceSpanningTree : acZeroImpedanceSpanningTree;
+    }
+
     /**
-     * Create the subgraph of zero-impedance LfBranches and their corresponding LfBuses
-     * The graph is intentionally not cached as a parameter so far, to avoid the complexity of invalidating it if changes occur
+     * Create the subgraph of zero-impedance LfBranches and their corresponding LfBuses.
+     *
      * @return the zero-impedance subgraph
      */
-    public Graph<LfBus, LfBranch> createZeroImpedanceSubGraph() {
-        return createSubGraph(branch -> branch.isZeroImpedance()
+    private Graph<LfBus, LfBranch> createZeroImpedanceSubGraph(boolean dc) {
+        return createSubGraph(branch -> branch.isZeroImpedance(dc)
                 && branch.getBus1() != null && branch.getBus2() != null);
     }
 
-    public Graph<LfBus, LfBranch> createSubGraph(Predicate<LfBranch> branchFilter) {
+    private Graph<LfBus, LfBranch> createSubGraph(Predicate<LfBranch> branchFilter) {
         Objects.requireNonNull(branchFilter);
 
-        List<LfBranch> zeroImpedanceBranches = getBranches().stream()
+        List<LfBranch> filteredBranches = getBranches().stream()
                 .filter(branchFilter)
                 .collect(Collectors.toList());
 
         Graph<LfBus, LfBranch> subGraph = new Pseudograph<>(LfBranch.class);
-        for (LfBranch branch : zeroImpedanceBranches) {
+        for (LfBranch branch : filteredBranches) {
             subGraph.addVertex(branch.getBus1());
             subGraph.addVertex(branch.getBus2());
             subGraph.addEdge(branch.getBus1(), branch.getBus2(), branch);

src/main/java/com/powsybl/openloadflow/network/impl/AbstractLfBranch.java

@@ -43,21 +43,28 @@ public abstract class AbstractLfBranch extends AbstractElement implements LfBran
 
     protected boolean voltageControlEnabled = false;
 
-    protected boolean spanningTreeEdge = false;
+    protected boolean dcSpanningTreeEdge = false;
+
+    protected boolean acSpanningTreeEdge = false;
 
     protected Evaluable a1;
 
     private ReactivePowerControl reactivePowerControl;
 
-    protected boolean zeroImpedance;
+    protected boolean dcZeroImpedance = false;
+
+    protected boolean acZeroImpedance = false;
 
     protected AbstractLfBranch(LfNetwork network, LfBus bus1, LfBus bus2, PiModel piModel, LfNetworkParameters parameters) {
         super(network);
         this.bus1 = bus1;
         this.bus2 = bus2;
         this.piModel = Objects.requireNonNull(piModel);
         this.piModel.setBranch(this);
-        zeroImpedance = isZeroImpedanceBranch(piModel, parameters);
+        if (!parameters.isMinImpedance()) {
+            dcZeroImpedance = isZeroImpedanceBranch(piModel, true, parameters.getLowImpedanceThreshold());
+            acZeroImpedance = isZeroImpedanceBranch(piModel, false, parameters.getLowImpedanceThreshold());
+        }
     }
 
     protected static List<LfLimit> createSortedLimitsList(LoadingLimits loadingLimits, LfBus bus) {
@@ -247,18 +254,23 @@ public double computeApparentPower2() {
     }
 
     @Override
-    public boolean isZeroImpedance() {
-        return this.zeroImpedance;
+    public boolean isZeroImpedance(boolean dc) {
+        return dc ? dcZeroImpedance : acZeroImpedance;
     }
 
     @Override
-    public void setSpanningTreeEdge(boolean spanningTreeEdge) {
-        this.spanningTreeEdge = spanningTreeEdge;
+    public void setSpanningTreeEdge(boolean dc, boolean spanningTreeEdge) {
+        if (dc) {
+            dcSpanningTreeEdge = spanningTreeEdge;
+        } else {
+            acSpanningTreeEdge = spanningTreeEdge;
+        }
     }
 
     @Override
-    public boolean isSpanningTreeEdge() {
-        return this.spanningTreeEdge;
+    public boolean isSpanningTreeEdge(boolean dc) {
+        network.updateZeroImpedanceCache(dc);
+        return dc ? dcSpanningTreeEdge : acSpanningTreeEdge;
     }
 
     @Override
@@ -286,18 +298,24 @@ public boolean isConnectedAtBothSides() {
     }
 
     @Override
-    public void setMinZ(boolean dc, double lowImpedanceThreshold) {
-        if (piModel.setMinZ(lowImpedanceThreshold, dc)) {
-            LOGGER.trace("Branch {} has a low impedance, set to min {}", getId(), lowImpedanceThreshold);
+    public void setMinZ(double lowImpedanceThreshold) {
+        if (piModel.setMinZ(lowImpedanceThreshold, true)) {
+            LOGGER.trace("Branch {} has a low impedance (dc = true), set to min {}",
+                    getId(), lowImpedanceThreshold);
+            dcZeroImpedance = false;
+        }
+        if (piModel.setMinZ(lowImpedanceThreshold, false)) {
+            LOGGER.trace("Branch {} has a low impedance (dc = false), set to min {}",
+                    getId(), lowImpedanceThreshold);
+            acZeroImpedance = false;
         }
-        this.zeroImpedance = false;
     }
 
-    private static boolean isZeroImpedanceBranch(PiModel piModel, LfNetworkParameters parameters) {
-        if (parameters.isDc()) {
-            return FastMath.abs(piModel.getX()) < parameters.getLowImpedanceThreshold();
+    private static boolean isZeroImpedanceBranch(PiModel piModel, boolean dc, double lowImpedanceThreshold) {
+        if (dc) {
+            return FastMath.abs(piModel.getX()) < lowImpedanceThreshold;
         } else {
-            return piModel.getZ() < parameters.getLowImpedanceThreshold();
+            return piModel.getZ() < lowImpedanceThreshold;
         }
     }
 }